


Setting Boundaries

by idontevenknowugh



Series: Black and Blue [5]
Category: Undertale (Video Game)
Genre: Awkward Sex, Dom/sub, Explicit Consent, M/M, Nervous Sex, Polyamory, RottenHoney, bound arms, discussions of non consentual kissing, he's still learning, mentions of bbqtacos, mentions of rottonberry, mentions of swapfellcest, not quite safewords, with art
Language: English
Status: Completed
Published: 2020-02-16
Updated: 2020-02-16
Packaged: 2021-02-28 05:02:56
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,186
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22748263
Author URL: https://archiveofourown.org/users/idontevenknowugh/pseuds/idontevenknowugh
Summary: Black's not one to suffer fools, but, for his lovers, he'll make an exception just this one time.
Relationships: Papyrus/Sans (Undertale), Sans/Sans (Undertale)
Series: Black and Blue [5]
Series URL: https://archiveofourown.org/series/682559
Comments: 7
Kudos: 39





	Setting Boundaries

**Author's Note:**

> Ah, I know it's been a while, but I still enjoy this series so so much! And this one was a lot of fun to write. I love people yelling at Stretch, haha. I hope you enjoy it too! <3
> 
> Art by the amazing [Lyco!](https://twitter.com/lycovore)! Thank you so much!

“What did you want to talk about?” Stretch asked, slouching, like always. Black reminded himself that Stretch wasn't doing a scene. They hadn't even discussed the prospect. Black was just judging him for his behavior because he didn't like Stretch all that much. He was here to see if that would—or could—change. 

“It has come to my attention that you have been… lacking for partners.” Black replied with no small amount of contempt. How had this idiot missed Blue’s very obvious pining? Stretch blanched and looked away.

“So he told you, like a good dog.” 

“Yes, he told me,” Black hissed, ignoring the fact that he had also witnessed it, kind of. “Of course he told me. He had nothing to hide. He said no, and you didn't listen.” 

It was heartening that Stretch flushed at that. He looked down at the ground miserably for a long moment before speaking. He clearly had no response for the accusation. 

“So you’re here to punish me?” He asked snidely, but his voice was soft. It didn't carry any real contempt in it. He was enough like Slim for Black to feel a little pang of regret, but he brushed it aside. Stretch was not Slim. Slim would have never behaved like that. 

“I'm here to, potentially, solve your problem,” Black explained with a sigh. He was too good to Slim and Blue. The temptation to dust Stretch was still present, but instead he was offering to sleep with him.

But only if he was a good boy. 

“What?” Stretch laughed. He gave Black a long look and burst out laughing fully. “Of course. Slim needs your permission, but you can just dally with whoever you like. I can see why you would like, though.” He flashed his tongue at Black. 

“Gross,” Black grimaced. “And don't be such a fucking idiot, for once. Of course I needed Slim’s permission,” and Blue’s he added silently, but they had agreed not to share that quite yet. “He asked me to do this.” 

Stretch’s laughter died. He stared at Black with his mouth hanging open. It made him look even more dimwitted. Black refused to explain further while he behaved like that, so he crossed his arms and waited. It was a little more complicated than that. Slim had somewhat taken pity on this poor fool after he calmed down, but the reason they had decided to try this was Blue. 

“Slim asked you to…?” Stretch sounded so confused Black was also tempted to take pity on him. He sighed and moved his hands to his hips as he gave Stretch a firm look. 

“Yes, he feels bad for you, even though I doubt you deserve it,” Stretch started to look like he had swallowed something sour. “But since I am not convinced you are worth his kindness, I will be training you, first.” 

“Training?” Stretch had apparently lost all coherency and was just parroting back what Black said to him. Black could work with that. 

“Yes,” Black raised one finger to instruct Stretch. “Before I let you near my brother,” or his own, “you will need to learn some manners.” 

Stretch looked skeptical, but he hadn't walked away yet. Too bad. 

“Do you want to do it?” Black asked. Stretch stared blankly at him, the expression morphing to amused confusion. 

“I thought you'd just order me to do it,” he commented, which unsettled Black slightly. “But yeah, I’m curious, I’ll do it.” 

“Good,” Black smirked. “Because you clearly have a lot to learn.” 

That brought a look of concern to Stretch’s face, making Black much more excited to move forward. 

——

“I expected a lot more chains,” Stretch said after looking around Black and Slim’s bedroom. “Some whips. Maybe an iron maiden. 

“What, exactly, would an iron maiden do to one of us?” Black asked. “Not that I would ever put Slim in a situation I couldn't keep tabs on.” He busied himself by checking that the bed sheets were perfectly straight as some residual shame flared up. 

“I think you're missing the point,” Stretch chuckled and wandered around the room, his hands in his hoodie pockets. 

“No, you are,” Black snapped. Stretch was already testing his patience. They hadn't even really started. “What we do isn't about chains or whips. It's about control, submission, and, woven through every aspect, trust.” 

“I guess,” Stretch didn't sound convinced. “I mean Slim would have to trust you quite a bit to let you do… whatever you do.”

“And I trust him to tell me his limits and stop the scene if I go past them.” 

“He has limits?” Stretch was incredulous. Black scowled at him and folded his arms. 

“Everyone does. Eventually, we might get to explore yours. But not until we establish trust.” Black pointed at the bed. “This is all we're using until we do. I need to be able to trust you to take this seriously, communicate with your partners, whoever they may be,” Stretch raised a browbone, “and understand consent.” 

“I know what consent is,” Stretch protested. 

“Then get out,” Black snapped and pointed at the door, “because you didn’t have Slim’s.” 

“It was just a kiss…” Stretch tried again. Black growled. Two strikes. 

“That he didn't want.” 

Stretch held Black’s sockets for only a moment before he looked down at the floor. There wasn't another snappy come back, just a nod of his skull that Black took as acknowledgement. 

“And you need to be able to trust that I will take every precaution I can with your safety, respect your limits, and give you complete control whenever we are in a scene,” Black concluded.

“What?” Stretch gave Black another blank look of confusion. Black sighed and sat down on the bed. 

“Rather than explain that, I think it would be more beneficial to show you. Thus, having sex.” 

“So romantic.” 

“If I don't think you can take this seriously, then tonight is all you get. Prove to me otherwise, and you can have everything you've ever wanted.”

“I want more than sex,” Stretch actually did turn serious. He stared at Black, and all the longing in him was on display. 

He wanted his brother, in much the same way his brother wanted him. Anyone else would have seen their mutual interest as enough and found a way to help them get together. Black was not most monsters, however. Stretch would hurt Blue over Black’s scattered dust. 

“Well then, let's see what we can give you,” Black dodged the implications of the statement. Stretch deflated, perhaps upset that Black didn't encourage him or show any sympathy. Of course he didn't have any sympathy for the spineless fool. 

“Here's what we’re going to do tonight.” Black pat the bed. Stretch hesitantly joined him. “I expect you to follow my every order.” Black held up his hand as Stretch opened his mouth. Stretch managed to get the message and closed it. 

“You are to tell me if you don't like any of it, at all. And tell me to stop at least once.”

“What… what if I don't want you to stop.” Black smiled, pretty sure Stretch wouldn't, once they got started. 

“Do it anyways,” Black told him, “understand?” 

“Sure, I guess,” Stretch shrugged. Black scowled at him. Maybe he was incapable of learning. Stretch noticed and grimaced. “Sorry, Yes… master?”

“I am not your master,” Black informed him strongly. It would be a long time before Stretch deserved to call him that, if ever. The thought of Stretch calling Blue master galled him even more. 

“Scoot back further onto the bed,” he instructed. Stretch did so, until just his feet were hanging off the end. Black decided it was good enough and straddled his lap. Stretch jumped as their legs touched. 

“You're serious,” Stretch breathed, awe in his voice. 

“I am,” Black nodded. “Are you? Do you still want to do this?” Stretch hesitated, and Black leaned away from him. 

“Wait, no, er- Yes. Yes, I want to,” Stretch flushed orange, a brighter shade than Slim’s. “You don't actually seem to like me,” he stated plainly. 

Black looked to the side for a moment before he caught himself and made himself look Stretch in the sockets. So he wasn't quite as dumb as Black thought.

“You aren't wrong,” Black admitted. “I am wary, given what I’ve seen of you.” Stretch flinched. “But bringing you into our play will make my partner,” partners, “happy.” 

“But you don't want to.” 

“It's more nuanced than that,” Black sighed and sat back on his heels and Stretch’s legs. He supposed it boded well, actually, that Stretch wanted to understand Black’s motives and his actual interest in what they were doing. It was better than him just continuing blindly for the sake of having sex. 

Black just wasn't entirely sure if he could explain without giving Blue away. 

“I want to make him happy,” Black started with the simplest truth. “And while I may not have picked you as a partner, I don't despise you so much that I won't try. Our way of being together isn't something you can just show up and do, however. Keeping it safe depends on everyone being on the same page. 

“Sure, Slim could teach you, he's probably the one who should be, but I wanted to lay some groundwork. For my own peace of mind.” Black tried to keep his voice neutral, but Stretch still looked upset.

“Like consent,” he said softly. 

“Yes,” Black acknowledged. 

“I’m surprised Slim wants me to join you,” Stretch laughed. “After what happened I expected to lose him as a friend, not be given a chance at a… a relationship.” 

“There's probably a lot that would surprise you,” Black commented and lifted himself back up onto his knees. “Would you still like to continue?” 

“Yes.” If anything, Stretch sounded less hesitant this time. Black nodded and cupped the side of his skull. They would need to take it slow as their emotions settled, so he kissed Stretch’s brow first. The interaction had done a surprising amount to sooth Black’s worries, as well. Stretch had some idea of what it meant that they were taking this chance on him. It wasn't his fault he didn't know the rest. 

Black trailed soft kisses along the top of Stretch’s skull, down between his sockets, and across the side opposite his hand. He felt Stretch’s jaw part slightly, but all that came out was a soft sigh. 

Black tapped his way down as he tilted Stretch’s face up until their mouths met. Stretch tensed underneath him. Black hoped he wasn't getting any strange ideas. 

He didn't let the kiss deepen. Stretch would have to earn that. Black moved on to kiss down Stretch’s jaw to his neck. Stretch obligingly made room for him to do so. 

Despite knowing that it was Stretch and not Slim, the bones were almost identical under his mouth. Black had to resist the urge to sink his teeth into the inviting vertebrae. Recalling Slim’s breathless moans of pain and pleasure didn't help. 

Stretch did moan, but he wasn't as reserved as Slim, for some reason. His moans were throaty and loud. Black didn't like them, but he had a feeling that was just because they were different. He had come to love the way Slim tried to hold back.

Now that Stretch was well distracted, Black pushed on his boundaries for the first time of the night. Without any warning, he shoved his hand down Stretch’s pants and grabbed his pelvis. 

“Wha- woah, stop!” Stretch sputtered, leaning back. Black immediately pulled his hand out and sat back, giving Stretch room. Stretch’s startled stare softened slowly into recognition, and he let out a big sigh.

“Well, you got me to say stop,” he laughed softly.

“Indeed,” Black almost felt a little bad for scaring him like that. “That was good.”

Stretch didn’t look all that happy with the praise, but he sat back up again. He gave Black a look and then shimmied his pants down as far as they would go.

Black moved off his legs so he could remove them completely. While he did, he slipped out of his own shorts, folding them and setting them aside. Stretch stared at his pelvis. It had a faint purple blush to it, just the very beginnings of arousal. In comparison, Stretch’s pelvis was glowing brightly.

“Lay down properly,” Black ordered, waving his hand. He untied his bandana and stripped off his shirt while Stretch complied. Black climbed onto the bed to find Stretch holding the hem of his hoodie down over his pelvis. Which did nothing to hide the continued glow. 

“Do you want to stop?” Black asked, staring at Stretch. He was already blushing, but he looked extra sheepish at that. He glanced down at Black’s pelvis and shook his head.

“Then we’re going to need that,” Black informed him, pointing at the covered pelvis. Stretch’s annoyed grimace only barely broke through before he was back to looking anxious.

“You aren't interested,” Stretch blurted suddenly, startling Black. He looked down at his own pelvis. There was a bit of warmth there, but nothing like Slim could draw forth. 

“How many times do we have to go over this,” Black crosses his arms. “I am hesitant, and that's not exactly going to rev me up. You could help by, I don't know, participating, rather than whining about it. I assumed you had had sex before, but it's a two way street. Entice me!” Stretch’s jaw hung open. 

“You're going to have to work for what you want,” Black leaned forward and held his face directly in front of Stretch’s. He could feel his temper rising. “Stop waiting for it to fall into your lap.”

Stretch nodded, a small scared movement that Black found extremely gratifying. He wasn't sure Stretch really understood what he was talking about, but teaching him to be a responsible, respectful, and attentive partner was a step in the right direction, even if he was a terrible sub.

At least he let go of the hoodie. Black snatched the hem and pulled it up past Stretch’s skull. His muffled cry of surprise died off once Black got the neck free and Stretch could see again. It was a little too soon to have him blind during play, but Black couldn't resist leaving the hoodie around his arms. He sat back and gave Stretch a chance to protest. Stretch simply looked up at him, fidgeting slightly. 

That did something for Black. A coil of want settled into his pelvis. Slim often looked nervous, but he never looked that…innocent. There was something sultry in him. Stretch had none of that, one thing he shared with Blue. Black softened a little at the comparison. Blue was a smart monster, and he saw something in Stretch.

With a huff, Black grabbed Stretch’s cock and stroked it. Stretch arched up into his hands with a surprised grunt. He didn't stop Black this time, however. Letting his eyelights wander, Black imagined the length in his hand was a rusty orange. It wasn't fair to Stretch to be daydreaming of his brother with that hint of a blush, but Black wasn't ready to forgive him all of the way. Trust took time. 

Besides, the mental imagine helped, along with Stretch’s deep moans. There was just something so gratifying about him enjoying the barest touch from Black that much. Black was kneeling right above Stretch’s cock, so, when his pussy formed, the lips brushed the tip. 

“Ah!” Stretch gasped and arched up, pressing his cock harder into Black’s folds. Black jumped and quickly sat back so that their magic was no longer touching. 

His soul was hammering away, and he had to clench his hands to keep them from shaking. He glanced at Stretch, who had lifted his skull to see what Black was doing. He hadn't made any kind of move, so at least his self control wasn't that bad. Still, Black couldn't continue. 

“I—” Black started and hesitated. In front of Slim, Blue, or even Red to some extent, he would have easily stopped everything without this sense of weakness. However, he had insisted he was willing to do this, and now he was going to back out. 

“Red,” he said with a wry smile. Stretch only tilted his skull in confusion. “We need to stop,” he clarified. Stretch’s baffled stare hardened into anger. Black clenched his jaw to ward off a flinch. 

“After all of that?” Stretch exclaimed, inching his trapped arms behind him so he was propped up on his elbows. “All that shit about working for it, and then just… nothing?” 

“Yes,” Black snapped. “You aren't owed sex just because you tried for five minutes.”

“But you said you were going to,” Stretch complained. 

“So what? You aren't ever entitled to sex, and if you guilt—!” Black swallowed Blue’s name before he could make this even worse. He was furious at the thought of Stretch pressuring Blue to continue. Blue was too eager to please, and if it was Stretch, of all monsters… 

“Sorry…” Stretch said, looking abashed, at least. Black took a moment to calm his soul. That’s exactly why he was doing this. Stretch needed to learn how to respect his partner. Blue’s confidence was growing, and, someday, he was going to confess. Black’s soul ached with bitterness. He couldn’t stop it, and it wasn’t his place to try. The most he could do was take steps to protect Blue when he did. 

“Sometimes, sex doesn’t work out,” Black said briskly, climbing off the bed and retrieving his clothes. “Try not to be a dick about it.” 

“Alright,” Stretch huffed. He opened his mouth like he had more to say, but stopped and closed it again. He pulled his hoodie over his skull and tugged it back down before looking around for his pants. Black tossed them to him, and they both put their clothes on in silence. 

“So I guess I fail?” Stretch said hesitantly as he finished dressing and started fiddling with his clothing instead. Black sighed and adjusted his shorts so they sat comfortably around his hips. 

“You messed up, a lot,” that got a satisfying flinch from Stretch, “but you showed a willingness to learn. I will contact you with the time of our next training session.” 

“Wait…really?” Stretch straightened and turned a shocked stare at Black. 

“I’m not looking for ways to get rid of you. I have more honor than that,” Black snapped, despite the fact that it had crossed his mind, repeatedly. 

“Thank you,” Stretch said, and it sounded as if he meant it. Black wasn’t sure he could keep his composure, so he simply walked over to the bedroom door and opened it, holding it for Stretch. 

He was mad that Stretch had shown said willingness and had accepted Black’s lessons so easily. Whether or not they would take was another matter, but for now Black was stuck with him. 

And someday he was going to lose Blue to him.


End file.
